Disclosed herein is a method for printing and viewing transparent ink on a substrate for purposes of accessing image quality drivers, such as drop placement, either manually or automatically. The method provides the ability to detect and inspect for missing or misaligned ink jets, registration errors, and jetting quality attributes, of jets that are utilizing clear ink, without any special substrates or expensive test equipment.
More particularly disclosed is a method for printing comprising disposing a first ink onto a substrate to form a first ink layer on the substrate in an area where a diagnostic image will be printed; disposing a second ink onto the first ink layer to form a second ink diagnostic image, wherein the first ink has a first color density and the second ink has a second color density that is different from the first ink color density, wherein the second ink drops hit and displace the first ink creating density holes which provide a density contrast; curing or drying after forming the second ink diagnostic image; wherein the second ink diagnostic image is rendered visible by the contrast between the second ink diagnostic image and the substrate as compared to the first ink layer and the substrate.
Ink jet printers have print heads that operate a plurality of jets that eject liquid ink onto an image receiving member. The ink may be stored in reservoirs located within cartridges installed in the printer. Such ink may be aqueous, oil, solvent-based, or radiation (such as ultra violet or electron beam) curable ink, or an ink emulsion.
A typical full width scan inkjet printer uses one or more print heads. Each print head typically contains an array of individual nozzles for ejecting drops of ink across an open gap to an image receiving member to form an image. The image receiving member may be a continuous web of recording media, a series of media sheets, or the image receiving member may be a rotating surface, such as a print drum or endless belt. Images printed on a rotating surface are later transferred to recording media by mechanical force in a transfix nip formed by the rotating surface and a transfix roller. In an inkjet print head, individual piezoelectric, thermal, or acoustic actuators generate mechanical forces that expel ink through an orifice from an ink filled conduit in response to an electrical voltage signal, sometimes called a firing signal. The amplitude, or voltage level, of the signals affects the amount of ink ejected in each drop. The firing signal is generated by a print head controller in accordance with image data. An inkjet printer forms a printed image in accordance with the image data by printing a pattern of individual ink drops at particular locations on the image receiving member. The locations where the ink drops land are sometimes called “ink drop locations,” “ink drop positions,” or “pixels.” Thus, a printing operation can be viewed as the placement of ink drops on an image receiving member in accordance with image data.
In order for the printed images to correspond closely to the image data, both in terms of fidelity to the image objects and the colors represented by the image data, the print heads must be registered with reference to the imaging surface and with the other print heads in the printer. Registration of print heads is a process in which the print heads are operated to eject ink in a known pattern and then the printed image of the ejected ink is analyzed to determine the orientation of the print head with reference to the imaging surface and with reference to the other print heads in the printer. Operating the print heads in a printer to eject ink in correspondence with image data presumes that the print heads are level with a width across the image receiving member and that all of the inkjet ejectors in the print head are operational. The presumptions regarding the orientations of the print heads, however, cannot be assumed, but must be verified. Additionally, if the conditions for proper operation of the print heads cannot be verified, the analysis of the printed image should generate data that can be used either to adjust the print heads so they better conform to the presumed conditions for printing or to compensate for the deviations of the print heads form the presumed conditions.
Analysis of printed images is performed with reference to two directions. “Process direction” refers to the direction in which the image receiving member is moving as the imaging surface passes the print head to receive the ejected ink and “cross-process direction” refers to the direction across the width of the image receiving member. In order to analyze a printed image, a test pattern needs to be generated so determinations can be made as to whether the inkjets operated to eject ink did, in fact, eject ink, and whether the ejected ink landed where the ink would have landed if the print head was oriented correctly with reference to the image receiving member and the other print heads in the printer.
Systems and methods exist for detecting ink drops ejected by different print heads, inferring the positions and orientations of the print heads, and identifying correctional data useful for moving one or more of the print heads to achieve alignment acceptable for good registration in the printing system. The ink drops are ejected in a known pattern, sometimes called a test pattern, to enable one or more processors in the printing system to analyze image data of the test pattern on the ink receiving substrate for detection of the ink drops and determination of the print head positions and orientation.
In some inkjet printing systems, print heads are configured to eject a clear ink onto the ink receiving member. This clear ink is useful for adjusting gloss levels of the final printed product and to provide a protective layer over printed areas. Clear, or transparent, ink can be used for at least two different applications in radiation curable, such as UV (ultra-violet) curable, printing systems. In one application, a clear primer is placed beneath all other images to improve adhesion of the other images to the substrate. In another application, a clear overcoat is printed on top of all of the other ink layers to add gloss and smooth image artifacts.
One issue that arises from the use of clear ink, however, is the difficulty in detecting drops of clear ink ejected onto an ink receiving member within an imaging system. Because the clear inks do not image well, the known systems and methods for aligning print heads do not enable the clear ink drops to be detected and the positions and orientations of the print heads ejecting clear ink to be inferred.
Techniques to measure ink drop placement are typically scanner based and measure the density difference of colored ink drops. With clear ink, there is no density differential to measure.
When radiation curable ink printing systems are being set up or debugged, it is important for the operator or an automatic imaging system to be able to see the clear ink. Viewing images is commonly done to ensure all jets are functional and to align the print head with respect to the printing plane as well as other print heads in the system. Prints must be viewed at angles in order to see the reflection of the clear ink. Since the operator can only see the outline of the ink at the correct angle, this method requires skill to be able to fully analyze all drops across the image. There are camera systems that can capture adequate images of clear ink with proper lighting on specialty papers, but this technology is costly and not readily available at printing sites.
U.S. Pat. No. 7,690,746, which is hereby incorporated by reference herein in its entirety, describes in the Abstract thereof a method for detecting a defect in an inkjet print head for printing a substantially clear ink includes including an ultraviolet or infrared sensitive material in the substantially clear ink, marking a test image on a substrate by jetting the substantially clear ink through one or more jets of the inkjet print head to be evaluated, exposing the test image to activating radiation having a wavelength to which the included ultraviolet or infrared sensitive material responds. During or following the exposing, the test image is evaluated with an image sensor, and whether the inkjet print head or any one of the one or more jets thereof being evaluated is defective is then determined based on the evaluation. A system for the method is also set forth.
U.S. Pat. No. 8,506,038, which is hereby incorporated by reference herein in its entirety, describes in the Abstract thereof a method that enables an operator to detect misalignment of print heads that eject clear ink in an inkjet printer. The method prints a first test pattern with a first color of ink and then prints a second test pattern of clear ink on top of the first test pattern. The ink of the first and the second test patterns is then spread to enable the clear ink to be dispersed in interstitial spaces in the ink of the first color. An operator is then able to detect the spatial relationship of predetermined marks in the second test pattern to predetermined marks in the first test pattern. The predetermined marks of the first and second test patterns are arranged to enable an operator to detect a misalignment distance and the inkjet printer uses the misalignment distance entered by the operator to adjust the alignment of the print heads that eject clear ink.
U.S. Pat. No. 8,602,518, which is hereby incorporated by reference herein in its entirety, describes in the Abstract thereof a test pattern printed by print heads in an inkjet printer enables image analysis of the test pattern that identifies positions of the print heads and the inkjets operating in the print heads. The test pattern includes a plurality of arrangements of the dashes, each arrangement of dashes having a predetermined number of rows and a predetermined number of columns, each dash in a row of dashes in the arrangement of dashes being separated by a first predetermined distance and each dash in a column of dashes in the arrangement of dashes being separated by a second predetermined distance, each dash in a column of an arrangement of dashes being ejected by a single inkjet ejector in a print head of the inkjet printer, and a plurality of unprinted areas interspersed between the plurality of arrangement of dashes.
Currently available diagnostic methods are suitable for their intended purposes. However a need remains for improved methods for viewing clear ink. Further, a need remains for an improved method for viewing clear ink disposed on a substrate without the need for complex and expensive hardware.
The appropriate components and process aspects of the each of the foregoing U. S. Patents and Patent Publications may be selected for the present disclosure in embodiments thereof. Further, throughout this application, various publications, patents, and published patent applications are referred to by an identifying citation. The disclosures of the publications, patents, and published patent applications referenced in this application are hereby incorporated by reference into the present disclosure to more fully describe the state of the art to which this invention pertains.